I think the “new experience” mobile Plex app has not yet gotten back the support for music libraries – so the library not showing up in the regular Android mobile app is to be expected (for now, Plex have announced this will be added back).
As for Plexamp… can you verify how they’re checking the library isn’t there for them?
Library selection should be possible by clicking on the library name at the top center of the home screen or via Settings > Source .
